Years ago, patients who underwent a bilateral medial temporal lobectomy were found to have developed anterograde amnesia. Surgeons now only remove one lobe if necessary, since the loss of one will still allow nearly normal memory function. However, it was still sometimes found that the removal of only one of the medial temporal lobes caused anterograde amnesia. Autopsies later revealed that such people had already experienced damage to the other hemisphere of the hippocampal system. The Wada test is designed to prevent such a surgical mistake. The procedure is as follows. The hemisphere that is to be operated on is administered with an anaesthetic (injected into the carotid artery). If the patients can still succeed in memory and learning tasks, then the conclusion is that they must have a functioning hippocampal system in the opposite hemisphere of the brain. Therefore, surgery can proceed.

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